help! diagnose tranny!

i kinda refuse to believe what the mech said about tranny replacement. i hope i can find an alternative way to fix it.
this is the situation:
i have an 88' 190E 2.6, with a 300E 3.0 motor swaped, i don't khow if they replace the tranny, i don't know where to look for numbers.
i'm experiencing neck snapping jerk every time i reach 4K rpm, any gear on 2, 3, and D they all jerks at 4K rpm.
can anyone please tell me why its acting like this?
redline is a little over 6K but it just wont let me pass over 4K
the mech told me i need tranny replacement which is about $1500
my car still runs good below 4K rpm, but whats the use of having 3.0 if i can't max it
on freeway, i can only run a little above 80mph, over that it will horribly jerks and its scary!!!
so please if anybody have an idea whats the cause, please say something.
all thoughts are apreciated.
thank you so much,
jds